In the field of communications, conventional microswitches (i.e. those used in microelectronics) are very widely used. They are useful in signal routing, impedance-matching networks, amplifier gain adjustment, and so on. The frequency bands of the signals to be switched can range from several MHz to several dozen GHz.
Conventionally, microelectronic switches have been used for these RF circuits, which switches enable circuit electronics integration and have a lower production cost. In terms of performance, however, these components are rather limited. Thus, silicon FET switches can switch high-power signals at low frequencies, but not at high frequencies. MESFET (Metal Semiconductor Field Effect Transistor) switches made of GaAs or PIN diodes work well at high frequencies, but only for low-level signals. Finally, in general, above 1 GHz, all of these microelectronic switches have a significant insertion loss (conventionally around 1 to 2 dB) when on and rather low insulation in the open state (from −20 to −25 dB). The replacement of these conventional components with MEMS (Micro-Electro-Mechanical-System) microswitches is therefore promising for this type of application.
Owing to their design and operation principle, MEMS switches have the following characteristics:                low insertion losses (typically lower than 0.3 dB),        high insulation in the MHz to millimetric range (typically over −30 dB),        no response nonlinearity (IP3).        
Two types of contact for MEMS microswitches are distinguished: ohmic contact and capacitive contact. In the ohmic contact switch, the two RF tracks are contacted by a short circuit (metal-metal contact). This type of contact is suitable both for continuous signals and for high-frequency signals (greater than 10 GHz). In the capacitive contact switch, an air space is electromechanically adjusted so as to obtain a capacitance variation between the closed state and the open state. This type of contact is particularly suitable for high frequencies (greater than 10 GHz) but inadequate for low frequencies.
Several major actuation principles for MEMS switches are distinguished.
Thermal actuation microswitches, which can be described as standard, are non-bistable. They have the advantage of a low actuation voltage. They have several disadvantages: excessive consumption (in particular in the case of mobile telephone applications), low switching speed (due to thermal inertia) and the need for a supply voltage to maintain contact in the closed position.
Electrostatic actuation microswitches, which can be described as standard, are non-bistable. They have the advantages of a high switching speed and a generally simple technology. They have problems of reliability, in particular in the case of low actuation voltage electrostatic switches (structural bonding). They also require a supply voltage in order to maintain contact in the closed position.
Electromagnetic actuation microswitches, which can be described as standard, are non-bistable. They generally operate on the principle of the electromagnet and essentially use iron-based magnetic circuits and a field coil. They have several disadvantages. Their technology is complex (coil, magnetic material, permanent magnet in some cases, etc.). Their consumption is high. They also require a supply voltage in order to maintain contact in the closed position.
Two configurations for moving the contact are differentiated: a vertical movement and a horizontal movement.
In the case of a vertical movement, the movement occurs outside the plane of the RF tracks. The contact occurs over the top or over the bottom of the tracks. The advantage of this configuration is that the metallization of the contact pad is easy to perform (flat deposit) and, therefore, the contact resistance is low. However, this configuration is poorly adapted for performing the function of dual contact switch. The contact over the top is indeed difficult to obtain. It is generally achieved by using a contact on the cap. This configuration also has poor integration compatibility. Indeed, for resistive switches, tracks and contacts with gold metallization are conventionally used (good electric properties, no oxidation). However, this metal is not integration compatible, even though it has been used since nearly the beginning of the technology for this type of configuration. There is no possible optimisation of the contact. Its surface can only be planar. The stiffness of the beam forming the contact is poorly controlled. This stiffness is conditioned by the final form of the beam which is dependent on the topology of a sacrificial layer which is itself dependent on the form and thickness of the tracks located below. The beam profile is generally irregular, which substantially increases the stiffness of the switch and therefore its actuation conditions.
In the case of horizontal movement, the movement takes place in the plane of the tracks. The contact takes place on the side of the tracks. This configuration is suitable for dual contact, with a symmetrical actuator. The “gold” metallization can be performed in the very last technological step. All of the preceding steps can be compatible with the production of integrated circuits. The form of the contact is determined in the photolithography step. For example, it is possible to have a round contact so that the contact occurs at one point and so as to thus limit the contact resistance. The form of the beam is determined in the photolithography step. Its stiffness is therefore well controlled. However, the metallization on the side is delicate. The contact resistance can therefore be poorly controlled. This configuration is unsuitable for electrostatic actuation due to the significantly-reduced opposing actuation surfaces.
The number of equilibrium states is another characteristic of the movement of the switches. In the standard case, the actuator has only one equilibrium state. This means that one of the two states of the switch (switched or unswitched) requires a continuous voltage supply in order to hold it in position. The interruption of the excitation causes the switch to move back to its equilibrium position.
In the bistable case, the actuator has two distinct equilibrium states. The advantage of this mode of operation is that the two “closed” and “open” positions of the switch are stable and do not require a power supply when there is no switching from one state to the other.